Since composite materials have not only improved mechanical and thermal properties but also enhanced dimensional stability and wear resistance as compared with conventional materials, they are widely used in the fields of polymers, metals, ceramics, and all related materials thereof. A number of studies on nanocomposites in which a reinforcement is added to a polymer are being actively undertaken in order to markedly improve the strength, stiffness, efficiency in inhibiting permeability to gases and liquids, flame retardance, wear resistance, high-temperature stability of the polymer, without damage to the impact resistance, ductility and transparency of the polymer.
Such reinforcements that are added to polymers are glass fibers, carbon fibers, clays, and the like. In the mid-1970's, the first nanocomposite prepared from a mixture of a polymer and a clay was developed by researchers from Toyota, who prepared a nylon 6/clay nanocomposite by filling nylon 6 with a clay and manufactured a timing belt box for a vehicle using the nanocomposite.
Clay exists as a laminate of layered silicates as inorganic minerals in nature. Each silicate layer is in the form of a broad plate with a dimension of 1 nm (in thickness)×1 μm (in width) and 1 μm (in length). The high aspect ratio and large surface area of clay silicate layers allow the clay to act as an effective reinforcement upon mixing with a polymer. Exfoliation of the laminated silicate layers enables preparation of a nanocomposite having superior physical properties. However, a disadvantage of clay-polymer composites prepared using clay as a filler is that direct exfoliation and dispersion of a clay in a polymer resin are difficult due to van der Waals attractive forces between the clay layers. Thus, several attempts have been made to intercalate a polymer between clay layers in order to exfoliate the layers from one another and to disperse the clay in the polymer on a molecular level.
To this end, various processes for introducing a polymer into clay layers are currently utilized, for example, a solution-intercalation process wherein a polymer in a liquid state is intercalated between clay layers, a in-situ-polymerization process wherein monomers of a polymer are intercalated between clay layers and are then polymerized, a melt-intercalation process wherein a mixture of a molten polymer and a clay is used in such a manner that the molten polymer is intercalated between the clay layers, etc. The clay used herein is commonly pre-treated with an alkyl ammonium so that the spacing between the clay layers is expanded to facilitate the introduction of the polymer between the layers.
The solution-intercalation process has the problem that the polymer is not sufficiently intercalated between the clay layers. No matter what the polymer is intercalated between the clay layers, the polymer expands the interlayer spacing but fails to exfoliate the clay layers. Accordingly, the intended improvement in physical properties is not achieved.
The melt-intercalation process has a limitation in that the polymer should have a processing temperature not higher than 200° C. The reason for this limitation is that since organic substances, such as alkyl ammonium, contained inside the clay are degraded at a temperature exceeding 200° C. the affinity of the polymer for the clay is deteriorated. In addition, since the layered structure of the clay collapses at high temperature, the interlayer spacing becomes likely to be narrow, making the permeation of the polymer into the layered structure difficult. As in the solution-intercalation process, even if the polymer is intercalated between the clay layers, exfoliation effects of the layers are unsatisfactory.
Korean Patent Laid-open No. 2002-17569 discloses a method for preparing a polyurethane containing clay dispersed in the polymer by mixing and reacting a quaternary ammonium salt-treated clay, an isocyanate compound and a polyol.
However, this process has the drawbacks that the polyurethane cannot be intercalated between the clay layers or cannot exfoliate the clay layers. That is, since the clay and the polyurethane remain only in the form a mixture, no improvement in the physical properties of the polyurethane is expected.
Further, there has been an attempt to intercalate a polyurethane between clay layers to exfoliate the clay layers by dispersing a clay in a long-chained polyol containing hydroxyl groups to intercalate a portion of the polyol between the clay layers, followed by reaction with an isocyanate compound to form a polyurethane. According to this attempt, however, since the surface of the polyol is very weakly bonded to the clay layers by intermolecular attractive forces, such as hydrogen bonding, the polyurethane prepared based on the polyol shows a very weak bonding force with the clay surface due to the intermolecular attractive forces. Accordingly, the interaction between the clay surface and the polyurethane is insufficient to break the bonding force between the clay layers. For this reason, little or no exfoliation of the layers takes place and the preparation of a nanocomposite is impossible, leading to a negligible improvement in physical properties.
On the other hand, a foamed clay-polymer composite prepared using clay as a reinforcement and a foaming agent shows poor physical properties when compared to the pure polymer foam. This is because clay and clay layers are agglomerated by blowing gas bubbles generated by the action of the foaming agent and hence serve as impurities, rather than due to incomplete exfoliation of the clay.